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Electroluminescent devices based on blended polymeric thin films

Abstract

This thesis is concerned with the characteristics of organic light-emitting devices based on the polymer poly(2֊methoxy֊5-(5'-ethyIhexyloxy)֊p֊phenylenevinylene) (MEH-PPV).The emissive layer of each device was formed either from pure MEH-PPV or from a blend of MEH-PPV with an electron-transporting small molecule material. A film of either indium tin oxide (ITO) or aluminium-doped zinc oxide (AZO) on glass was used to provide a transparent anode. The pure or blended emissive material was spin-coated from solution onto the anode. Calcium or aluminium cathodes were deposited by thermal evaporation onto the emissive film. The cutrent-voltage, light-voltage, quantum efficiency and stability characteristics of the devices were obtained using purpose-designed measurement equipment. These characteristics were influenced, amongst other factors, by the blend composition. The devices with an aluminium cathode and an emissive layer formed from 50 % MEH-PPV and 50 % PDPyDP had among the best characteristics. Compared with the equivalent unblended devices they averaged 40 times greater light emission, 35 times higher efficiency, and they emitted light for more than 18 days in place of tens of minutes. The half life of the most stable devices was 63 һ which, though much less than the tens of thousands of hours claimed by commercial developers, represents an increase of 30 times that of an unblended device. The increased light emission and quantum efficiency data resulted from the enhanced earner injection and transport provided by the electron transport material. It is suggested that the increased stability was caused by the small molecule material performing the role of a nano-encapsulation around each polymer chain. That is to say, the data suggest that each polymer chain was individually protected from the effects of contaminants such as oxygen and water vapour by the small molecule within the film .The choice of anode material also affected the rate of degradation. The surface morphology of the anode material of devices which had previously been operated for extended periods were analysed using an AFM. Electromigration of ITO anodes to form sharp spikes was not uncommon, particularly in devices with short lifetimes. As-deposited AΖO generally had a smoother morphology than as-deposited ITO. Following device operation fewer electromigtation spikes were observed in AZO than ITO.